The advent of modern turbofan engines saw the introduction of a fundamental change in 3-shaft engine architecture, namely a change to a single skin high pressure (HP) turbine. The 3-shaft engine architecture requires three separate compressor offtakes to feed different temperature cooling air to each of the three turbines. In prior engines concentrically mounted cases have been used to achieve the separation and transfer of the three compressor cooling air flows.
One consequence of the change to a single skin casing has been the need to transport HP3 cooling air from the HP compressor to the HP turbine via large bore pipes routed around the outside of the engine core. The HP3 air delivery pipework into the intermediate pressure (IP) turbine is typically routed onto bosses on the HP-IP case. Owing to the circumferential locations of bearing support structural mount points and oil and air service pipes, this means that the high pressure air is typically directed straight onto the inter-platform gaps of the IP nozzle guide vane (NGV) outer platforms immediately below the bosses.
This system has various disadvantages. For instance:                There is uneven distribution of the cooling air, leading to localised hot spots and accompanying hardware deterioration.        In some arrangements where modulated HP3 air flow is introduced into the engine via independently controllable valves arranged in the HP3 air delivery system, the modulated and non-modulated air pipes may not be interspersed symmetrically, which results in over half the circumference of some of the pipes not receiving direct HP3 air input.        The current system encourages leakage flow of air past strip seals which are typically located between the IP NGV's by the blowing of cooling HP3 air straight onto them.        It also encourages heat pickup from the gaspath components owing to the airflow's direct impinging thereon. This can lead to deterioration of downstream components due to the delivery thereto of higher temperature cooling air.        It also encourages thermal cracking of gaspath components owing to the introduction of localised high thermal gradients.        
An example of one prior attempt at improving cooling air flow distribution within a gas turbine engine manifold is disclosed in U.S. Pat. No. 8,100,633B2 (United Technologies Corp.). The arrangements disclosed there involve the use of splash plates to direct radially incoming air from a cooling air conduit such that at least some of the cooling air spreads circumferentially within the manifold. However, these known splash plate arrangements are of only limited efficacy in providing improved cooling airflow distribution, and many, if not all, of the above-mentioned problems still remain to some degree.
Other distribution devices are described in U.S. Pat. No. 3,529,941, US2014212281, U.S. Pat. No. 5,239,816, EP1074694 and US2012087784.